The following summary information is a synopsis of and direct excerpts from the National Estuary Program Open File GBNEP-31, April 1993, Trends and Status of Wetlands and Aquatic Habitats in the Galveston Bay System, Texas by William A. White, Thomas A Tremblay, and E.G. Wermund, Jr., and Lawrence Handley.

Introduction

Status of Wetlands: 1989

Trends: 1950's to 1979 to 1989

Overall Trends and Their Probable Causes: 1950-1989

Estuarine and Palustrine Emergent Wetlands

Table 3 Status (1989) of major estuarine and palustrine systems

Table 4 Status (1989) of major estuarine and palustrine systems

Scrub-shrub and Forested Wetland

Description of Wetlands for the Virginia Point Area

Estuarine System

Palustrine System

Probable Cause of Wetland Change in Virginia Point area

Subsidence and Sea-Level Rise

Man-Induced Subsidence

Subsidence in the Houston area

Subsidence-Related Losses in Emergent Wetlands

Other Causes of Major Losses

Figure 9 Distribution of emergent habitats in the Virginia Point quad

 Background Information
Texas City/ Virginia Point

Introduction

Wetland and aquatic habitats are essential biological components of the Galveston Bay Estuarine System. Understanding the spatial and temporal distribution of these habitats is critical if they are to be effectively protected and managed. This report presents results of an investigation sponsored by the Galveston Bay National Estuary Program (GBNEP) to determine the trends and status of wetlands and aquatic habitats in the Galveston Bay system through aerial photographic analysis supported by field surveys.

Methods

Status and trends of wetlands in the Galveston Bay system were determined by analyzing the distribution of wetlands mapped on aerial photographs taken in the 1950's, 1979, and 1989. Wetlands for all maps were delineated on photographs through stereoscopic interpretation using procedures developed for the U.S. Fish and Wildlife Service (USFWS) National Wetlands Inventory program. Field reconnaissance was an integral part of the interpretation process. Wetlands were classified by systems (marine, estuarine, riverine, palustrine, lacustrine), subsystem (reflective of hydrologic conditions), and class (descriptive of vegetation and substrate). Data for 1979 and 1989 were additional classified by subclass (subdivisions of vegetated classes only), water-regime, and special modifiers.

More than 180 field sites were examined as part of the effort to characterize wetland plant communities and define wetland map units in the Galveston Bay system. Topographic surveys were conducted along several transects. County soil surveys were used to define and characterize soils and the various field check sites.

Table 1. Areal extent of wetland systems and uplands.

Figure1: Study area defined by 30 USGS 7.5 minute series and location of WETMAAP Texas City/Virginia Point site.

Table 2: Status (1989) of major estuarine and palustrine habitats.

Estuarine Intertidal Emergent Wetlands (Salt and Brackish Marshes)

22,200 acres (16% of vegetated wetlands and 17% of marsh habitats)

Most extensive distribution is along the Trinity River alluvial valley and inland of

Christmas, West, and East Bays

Extent of Vegetated Wetlands in 1989

Figure 3: Areal extent of vegetated wetlands in 1989

Upland Habitats in 1989

Figure 4: Aerial extent of upland habitats in 1989

Trends : 1950's to 1979 to 1989

There were gains and losses in wetlands form the 1950's to 1989, but the net trend was one of wetland loss. This downward trend is illustrated by acreages of 171,000 in the 1950's, 146,000 in 1979, and 138,600 in 1989 (table 2.4 and figure 2.5). The rate of loss, however, decreased over time from about 1,000 acres per year between 1953 and 1979, to about 700 acres per year between 1979 and 1989. The rate of loss for the period 1979 to 1989 would be lower (<500 acres/yr) if inaccuracies in wetland interpretation on the 1979 photographs are taken into account. In general, scrub-shrub habitats decreased in area from the 1950's to 1979 and 1989 while forested wetlands increased (table 2.3).

Overall trends and Their Probable Causes: 1950-1989

In analyzing trends, emphasis was placed on wetland classes and not on water regimes and special modifiers. Emphasis is placed on net losses in wetlands. Losses in wetland vegetation resulted from conversion of the wetlands to (1) open water and flats, (2) uplands, and (3) other wetlands classes. From the 1950's to 1989, there was a net loss in vegetated wetlands of 32,400 acres, which amounts to 19 percent of the vegetated wetland system that existed in the 1950's.

The area emergent wetlands (marshes) decreased from about 165,500 acres in the 1950's to about 130,400 acres in 1989, producing a total net marsh loss of approximately 35,100 acres, or 21 percent of the 1950's resource. Like vegetated wetlands, this amount of loss in emergent wetlands is thought to be on the high side; the actual lost is probably below 19 percent.

Net losses in vegetation occurred in 25 of the 30 quadrangles (quads) studied. The most substantial losses occurred in the southwest part of the study area and include Virginia Point, Hitchcock, Hoskins Mound, Texas City, and Sea Isle. Approximately 55 percent of the total losses of emergent vegetation in the Galveston Bay System occurred in these areas. The most extensive net loss, exceeding 5,000 acres, occurred in the Virginia Point quad in the inland margin of West Bay.

The causes of wetland loss include both natural and artificial processes. Among them are humanly induced subsidence and relative sea-level rise, and draining and filling of wetlands for agricultural, transportational, industrial, residential, and commercial purposes (table 2.4). Major losses in estuarine emergent wetlands (salt and brackish marshes) occurred as they were converted to open water and barren flats. Major losses in palustrine emergent wetlands (interior, or fresh marshes) resulted from their conversion to uplands.

Although the net loss in emergent wetlands (marshes) from the 1950's to 1989 encompassed about 35,000 acres, the gross loss, exclusive of offsetting gains in other areas, was considerably larger--approximately 88,500 acres. Conversion of emergent wetlands to open water and flats exceeded 26,400 acres, accounting for about 30 percent

Table 3: Areal extent of vegetated wetlands and aquatic beds in the 1950's, 1979, and 1989.

Table 4: Major causes of wetland losses from the 1950's to 1989.

of the total gross loss. There is evidence that humanly induced subsidence and associated relative sea-level rise was the major factor contributing to the conversion of marshes to open water and barren flats (table 2.4). Subsidence along active surface faults contributed to marsh loss in some areas.

Major losses in interior, or fresh, marshes occurred as large areas of palustrine emergent wetlands were transformed to uplands. The magnitude of this change is approximately 35,600 acres from the 1950's to 1989, and accounts for about 40 percent of the total gross loss in palustrine and estuarine emergent wetlands. The change from emergent wetlands in the 1950's to upland rangeland in 1989 encompassed 25,400 acres. Conversion of wetlands to urban upland areas amounted to 5,700 acres, and to cropland and pastureland, 3,600 acres (table 2.4). It appears that some changes are related to natural conditions, such as annual (and seasonal) changes in moisture levels, which affected photointerpretation, but a substantial amount of the changes appears to be due to draining of wetlands. This has been a common practice, especially from 1950's to 1970's. Approximately 33 percent of the gross loss is emergent wetlands is attributed to conversion of marshes to upland rangeland and cropland.

Scrub-Shrub and Forested Wetlands

The general trend in scrub-shrub wetlands for the 1950's to 1989 period was one of net loss. However, this trend was countered by forested wetlands, which had a significant net gain. Scrub-shrub wetlands decreased by approximately 850 acres, representing a loss of about 25 percent of the 1950's resource. Forested wetlands, on the other hand, increased in area by approximately 3,600 acres, an increase of about 1.8 times the 1950's area. Much of this gain in forested wetlands was due to (1) taller growth of shrubs and trees in areas previously mapped as scrub-shrub wetlands and (2) inconsistent delineation of forested wetlands on the different sets of photographs. Locally, losses in forested wetlands were due to alterations in hydrology.

Description of Wetlands for the Virginia Point Area.

The geologic framework of the Galveston Bay area consists of Modern-Holocene and Pleistocene systems including the modern wetland, or marsh and marsh-swamp systems (figure 2.6). The geomorphic features on which the various types of coastal wetlands have developed are the result of numerous interacting processes. Physical processes that influence wetlands include rainfall, runoff, fluctuations in the water table, streamflow, evapotranspiration, waves and longshore currents, astronomical and wind tides, storms and hurricanes, deposition and erosion, subsidence, faulting, and sea-level rise (table 2.5). These processes have contributed to the development of a gradational array of permanently inundated to infrequently inundated environments ranging in elevation from submerged lands of the estuarine system through the typographically higher wetland system, which grades upward from the astronomical-tidal zone through the wind-tidal zone to the storm-tidal zone.

Exchange of marine waters with bay-estuary-lagoon waters in the Galveston Bay system occurs primarily through two major tidal inlets, Bolivar Roads at the north end of Galveston Island, and San Luis Pass at its south end (figure 2.6). Additional exchange occurs at Rollover Pass, a narrow dredged channel at the east end of Bolivar Peninsula. The predominant sources of fresh-water inflow are the Trinity and San Jacinto Rivers (figure 2.6).

These numerous interacting processes in the Galveston Bay system have a major bearing on the location and composition of wetland plant communities.

Estuarine System

The estuarine system consists of many types of wetland habitats. Estuarine subtidal unconsolidated bottom (E1UBL), or open water, occurs in the numerous bays and in adjacent salt and brackish marshes. Unconsolidated shore (E2US) includes intertidal sand and mud flats and estuarine beaches and bars. Water regimes for this habitat range from irregularly exposed (E2USM), regularly flooded (E2USP) to irregularly flooded (E2USP).

Emergent areas closest to estuarine waters consist of regularly flooded (E2EM1N) salt-tolerant grasses (low salt and brackish marshes). These communities are mainly composed of Saprtina alterniflora, Distichlis spicata, Salicornia spp., Batis maritima, Juncas roemerianus, and Scripus maritimus in more saline areas. In brackish areas such as in the Trinitiy River delta at the head of Trinity Bay, species composition changes toward a brackish to fresh-water assemblage. At slightly higher elevations the irregularity flooded estuarine emergent wetlands (E2EM1P) (high salt and brackish marshes) include Spartina patens, Distichlis spicata, Spartina spartinae, Scripus maritimus, Borrichia frutescens, Astrer spp., and many others.

Table 5. Generalized characteristics of active coastal processes and conditions in the Galveston Bay area.

Figure 6. Natural systems in the Galveston Bay area.

Estuarine scrub-shrub wetlands (E2SS) are much less extensive than estuarine emergent wetlands. Representative plant species, which generally occur in irregularly flooded zones (E2SS1P) between emergent wetland communities and upland habitats, include Iva frutescens, Baccharis halimifolia, Sesbania drummondii, and Tamarix gallica.

Relative sea-level rise as used here refers to a rise in sea level with respect to the surface of the land, whether it is caused by actual sea-level rise or land-surface subsidence; the current general trend along the Texas coast, and in the Galveston Bay area, involves both of these processes

It is generally accepted that over the past century sea level has been rising on a worldwide eustatic basis at a rate of about 1 to 1.5 mm/yr, with a rate in the Gulf of Mexico and Caribbean region approximating 2.4 mm/yr. Adding "natural" compactional subsidence to these rates yields a relative sea-level rise that locally exceeds 10 mm/yr.

Man-Induced Subsidence

Subsidence due to withdrawal of ground water from an artesian aquifer results from a decrease of hydraulic pressure and attendant movement of water from clays to adjacent sands leasing to compaction of the clays. Most of the compaction is permanent because of the inelastic nature of the clay; thus even, with total recovery of artesian pressure, less than 10 percent rebound can be expected.

Transformation of emergent wetlands, or marshes, to areas of water and flats has occurred to some degree throughout the area. Total loss across all quads has exceeded 26,000 acres, accounting for about 30 percent of the total loss (gross loss excluding gains was about 88,500 acres) in the Galveston Bay system. Not all this loss can be attributed to subsidence, but throughout the region a whole, subsidence has been dominant as the major contributing factor to the replacement of emergent wetlands by open water and flats.

In Virginia Point, more than 3,600 acres of marshland was replaced by open water and mud flats between the 1950's and 1989. Significant subsidence has been documented in the area. Loss of marshland has been most extensive northwest and west of Jones Bay, where salt marshes have been converted to estuarine unconsolidated shore (E2US, intertidal flat) and estuarine unconsolidated bottom (E2SB, open water) (figure 2.8).

Additional losses occurred along the margins of Swan Lake (figure 2.8). Open water and flats also encroached into marshes in the adjacent Hitchcock quad. Approximately land-surface subsidence in the Virginia Point quad from 1906 to 1987 ranged from slightly less than 0.6 to 1.8 m (<2 to 6 ft). Estimated rates of subsidence in the area northwest of Jones Bay for the period of 1943 to 1987, exceed 14 mm/yr (0.05 ft/yr). This rate apparently was higher than rates of marsh aggradation in this area. On Galveston Island

Figure 8: Relationship between subsidence and losses in emergent wetlands (shaded) by conversion to water and barren flats in the Virginia Point quad. Contours (in feet) show amount of subsidence that occurred between 1906 and 1987.

across West Bay from Virginia Point, there is also evidence that subsidence rates locally exceed marsh aggradation rates.

Other Causes of Major Losses

Many losses and changes in wetland habitats are caused by process or activities other than subsidence. Among human activities in the Galveston Bay system that have a direct effect on wetlands and can be quantified to some degree through photoanalysis are construction of levees and dams to impound water; excavation (dredging) for purposes such as navigation, flood control, and mineral (sand) extraction; drainage for conversion to other land uses such as agriculture; and filling for purposes of spoil disposal or residential-commercial-industrial development.

Indirect losses due to such activities as chemical discharges, hydrologic alterations, and sediment diversion cannot adequately be determined through photoanalysis and will only be mentioned as possible contributing agents in some changes.

Activities causing direct changes assessed through photoanalysis can be classified into two board categories: (1) activities that convert emergent wetlands into flats and open water such as constructing impoundments and dredging (excavating) and (2) activities that convert wetlands into uplands such as draining, altering hydrology, and filling.

Virginia Point, in terms of total area and second to the Cove quad, was affected by impoundments and excavations. Nine-hundred acres of emergent wetlands (PEM, E2EM, and E2EM/FL) existing in the 1950's were replaced by water and faults at impounded and excavated sites (figure 2.9). Total wetland losses caused by channel dredging was larger than this because of associated filling of adjacent wetlands to produce upland sites foe urban development.

Conversion of Wetlands to Uplands

Emergent wetlands in the 1950's were identified as upland in 1989. The magnitude of this change is approximately 35,800 acres from the 1950's to 1989. The wetland system and class most heavily impacted by this type of change is the palustrine emergent wetland class

Figure 9: Distribution of emergent wetland habitats (shaded) in the 1950's that became excavated or impounded areas of open water and barren flats by 1989 in the Virginia Point quad. Excavated channels for existing or proposed residential developments surround Jones bay, and industrial impoundments characterize the area northwest of Swan Lake in the Texas City area.

by

W.A. White, T.A. Tremblay, and E.G. Wermund, Jr.

Bureau of Economic Geology, The University of Texas at Austin

and

L.R. Handley

National Wetlands Research Center, U.S. Fish and Wildlife Service